The invention relates to a cardiac pacemaker system of the type including a stimulation electrode adapted for being arranged in the heart; an output capacitor coupled to the stimulation electrode; a first circuit, coupled to the output capacitor, for generating a pulse following each stimulation pulse for at least one of reducing a residual charge of the output capacitor and eliminating an afterpotential following a stimulation pulse by the stimulation electrode; and a third circuit, for acquiring an evoked heart action from an electrical signal picked up by an electrode arranged in the heart.
For a long time, it has been a goal in the development of artificial cardiac pacemakers to verify the success of a heart stimulation through the measurement and evaluation of signals, which can be picked up in the heart on the basis of the evoked heart action via an electrode which is installed in the heart--and preferably the stimulation electrode itself.
The pickup of the electrical "response signal" of the heart after a stimulation is disturbed by the aftereffects of the stimulation pulse, which are caused by the polarization of the stimulated tissue, which can be reduced only when the recharging of the output (coupling) capacitor connected to the stimulation pulse is also eliminated.
This follows from the fact that the evoked potential, which indicates the success of the stimulation and is present at the heart approximately up to 300 ms after the stimulation, is superposed by an afterpotential in the order of magnitude of more than 10 mV. The afterpotential, which disturbs the effectiveness for recognition of the evoked potential, is caused by the effect of the stimulation electrode as an electrochemical electrode, from which results a saturation of the detection amplifier.
Apart from various circuits, with which attempts are made to eliminate the consequences of the afterpotential, an apparatus for the stimulation of the heart is known from EP-B1-0 000 989, wherein the disturbing afterpotential of the stimulation electrode is intended to be reduced in an accelerated manner by means of an additional, transistor-controlled resistor branch, which essentially short-circuits the Helmholtz capacity. The total time needed for the reduction of the disturbing afterpotential, however, is too long with customary electrodes to make possible an effective effectiveness recognition under all circumstances.